The object of the invention is a method for producing multi-layer filtering material, and a multi-layer filtering material. According to the method, at least one filtering layer of natural fibres, such as wood pulp fibres and/or plastic fibres and binder material and/or binder fibres is formed by using the dry forming technique and the filtering layer is bonded by heat.
In addition, the object of the invention is a multi-layer filtering material comprising at least one filtering layer of natural fibres, such as wood pulp fibres and/or plastic fibres and binder material and/or binder fibres, said filtering layer being formed by using the dry forming technique and being bonded by heat, said filtering material further comprising a strengthening layer formed of natural fibres, such as wood pulp fibres and/or plastic fibres and binder material and/or binder fibres, said strengthening layer being arranged to be in contact with the filtering layer.
At its simplest, the filtering material of air filters comprises of only one material layer. This kind of filtering material is known, for example, from U.S. Pat. No. 4,765,812. In efforts to improve the filtering characteristics, the filtering material is increasingly often formed of either at least two separate layers, which are bonded together, or of at least one finished layer, depending on the desired filtering characteristics.
The filtering material produced in this manner is used in the manufacture of different kinds of filters. To an increasing extent, as the standard of filtering properties required rises, there has been a tendency to go over to multi-layer solutions. The problem in this connection has been the additional production stage of joining two or more layers together or the additional finishing stage.
The function of the invention is to provide a method for producing a filtering material with good adjustable filtering characteristics and which is nevertheless advantageous in terms of manufacturing technology. This kind of multi-layer filtering material can be produced by the production method in accordance with the invention, characterised in that, in addition to the forming stage of the filtering layer, said method further comprises the steps of forming at least one strengthening layer of natural fibres, such as wood pulp fibres and/or plastic fibres and binder material and/or binder fibres, said strengthening layer being formed by using the dry forming technique and said layer being arranged so as to be in contact with said at least one filtering layer in order to form a unity, said unity being formed at the same time as the heat bonding of the filtering layer.
Preferably, this can be achieved by forming said at least one strengthening layer on one or both surfaces of the filtering layer. In practice, this can be done in such a way that when the filtering material web is being formed by the dry forming technique, one or more filtering layer/s are formed first, and on top of said filtering layer/s one or more strengthening layer/s are formed before the unity is bonded by heat. Alternatively, before forming the filtering layer/s, one or more strengthening layer/s are formed, on top of which first the filtering layer/s and then the strengthening layer/s are formed before the unity is bonded by heat. As mentioned above, the filtering layer can be formed of several layers one on top of another, depending on the desired filtering characteristics. The most essential characteristic of the strengthening layer is to improve the strength characteristics of the filtering material. In a multi-layer structure according to the invention, the filtering layer or layers are porous layers, giving good filtering characteristics, and with the help of one or more strengthening layer/s the material can be given the required strength properties.
A multi-layer filtering material according to the invention is characterised in that said at least one strengthening layer is formed by the dry forming technique, said strengthening layer being formed so as to be in contact with said filtering layer and bonded in connection with the heat bonding of the filtering layer, said strengthening layer forming a seamless unity with said at least one filtering layer. Thus the multi-layer filtering material in accordance with the invention is produced entirely by the dry forming technique in one continuous process without any additional production stages.